Conventionally, there are four-wheel drive vehicles equipped with a driving force distribution device for distributing driving force generated in a driving source such as an engine to main driving wheels and sub-driving wheels. In a four-wheel drive vehicle of this type, for example, when front wheels are the main driving wheels and rear wheels are the sub-driving wheels, the driving force generated in the driving source is transmitted to the driving force distribution device with a multi-plate clutch via a propeller shaft, as well as being transmitted to the front wheels via a front drive shaft and a front differential. And, the engagement pressure of driving force distribution device is controlled by supplying hydraulic fluid at a predetermined pressure from a hydraulic controller to the driving force distribution device. Thus, the driving force of driving source is transmitted to the rear wheels at a predetermined distribution ratio.
Also, as a hydraulic controller for controlling hydraulic pressure supplied to the multi-plate clutch of the driving force distribution device, conventionally there are hydraulic controllers as shown in Patent Documents 1 and 2. The hydraulic controllers shown in Patent Documents 1 and 2 include an electric oil pump to supply hydraulic fluid to a hydraulic chamber for pressing the multi-plate clutch and are configured to connect the electric oil pump and the hydraulic chamber with a hydraulic supply passage. In the hydraulic controller of Patent Document 1, the rotational speed of the electric pump is controlled such that the discharge value of the electric pump becomes the required operating pressure of the hydraulic clutch. In the hydraulic controller described in Patent Document 2, the motor drive of the electric pump is controlled so as to generate hydraulic pressure in accordance with the distribution ratio of driving force.
However, in the hydraulic controllers of Patent Documents 1 and 2, due to a configuration that supplies hydraulic pressure necessary for the hydraulic clutch by driving the electric pump, it is necessary to operate the electric oil pump constantly at the time of engagement of the hydraulic clutch. Therefore, when a brush motor is used as a motor to drive the electric oil pump, it is difficult to guarantee the durability of the motor (brush abrasion).
Further, in a system that controls the engagement pressure of the clutch by the hydraulic pressure of hydraulic fluid as mentioned above, the relationship between the engagement pressure and torque of the clutch has desirably linear characteristics. However, due to the presence of hysteresis characteristics of clutch torque, such linear characteristics may not be attained. As a countermeasure thereto, the characteristics of clutch torque has been conventionally needed to be improved by making changes to the hardware configuration such as the shape of clutch plate. However, such changes in the structure of clutch may lead to a cost increase and complexity of the structure of product.